1. Field of the Invention
The present invention relates to a process for preparing epitaxial compound semiconductors, and more particularly to a process for forming a single crystal film of compound semiconductor by epitaxial growth.
2. Description of the Prior Art
In preparing electroluminescence devices of compound semiconductors such as zinc sulfide (ZnS), molecular beam epitaxy (MBE) is known as a method which is excellent in film thickness controllability and quantity production efficiency for use in the production process including epitaxial crystal growth of the compound semiconductor on a semiconductor substrate and formation of an electrode. For example, for the crystal growth of ZnS by the MBE method, a simple element material of Zn, and a simple element material of S or sulfur hydride (H.sub.2 S) material are heated in Knudsen cells independently of each other to produce respective molecular (atopic) beams, which are supplied on a fully heated semiconductor substrate to effect crystal growth ("Growth of ZnS Bulk Single Crystals and Homoepitaxial Growth of ZnS by Molecular Beam Epitaxy," Extended Abstracts of the 19th Conference on Solid State Devices and Materials, Tokyo, 1987, pp. 247-250; "SINGLE CRYSTAL GROWTH OF ZnS BY THE METHOD OF GAS SOURCE MBE," Journal of Crystal Growth, 76(1987)440-448, North-Holland, Amsterdam).
Further the layers of multi-layer epitaxial growth crystals for semiconductor electroluminescence devices are doped with impurities for controlling the conduction type and luminescence color thereof. For example, when II-VI compound semiconductors such as ZnS are to be made n-type, the impurity elements to be used include aluminum (Al), gallium (Ga), indium (In), vII elements such as iodine (I), bromine (Br), chlorine (Cl) and fluorine (F), etc. Examples of impurity elements useful for making such semiconductors p-type are I elements such as lithium (Li), sodium (Na) and potassium (K), and V elements such as nitrogen (N), phosphorus (P), arsenic (As) and antimony (Sb). These semiconductors are of semiinsulating properties in the absence of dopants or when doped with a IV element such as silicon (Si) or germanium (Ge), or with the combination of one of the above-mentioned III and VII elements and one of the above I and V elements. Examples of impurity elements useful as dopants for providing luminescent centers are manganese (Mn) and Lanthanoids (rare-earth elements) including lanthanum (La), cerium (Ce), praseodymium (Pr), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).
According to the conventional MBE method, a substrate heated to a suitable temperature is irradiated with a molecular or atomic beam for forming a matrix or with such a beam as heated to a high temperature to grow a compound semiconductor single crystal, and is also irradiated with a molecular or atomic beam of impurity element at the same time for doping.
However, many of materials for the foregoing impurity elements (beam materials, for example, of zinc, sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, iodine, bromine, chlorine and fluorine) have a high vapor pressure of element, are low in depositability on the substrate which is heated to a suitable temperature (at least 300.degree. C.) required for the growth of compound semiconductors, and therefore encounter difficulties in growing high-quality single crystals of each of the compounds constituting compound semiconductors.
On the other hand, if compound semiconductors are grown at a high temperature which permits the growth of high-quality crystals of each of the compounds, the crystals will develop point defects such as vacancies leading to deep levels or complex defects and become contaminated with objectionable impurities. It is therefore desired to grow the crystals at the lowest possible temperature to avoid such serious drawbacks in respect of the characteristics of the semiconductor thin film.
Of the impurity elements or component elements of semiconductors, the metal elements (zinc, cadmium, aluminum, gallium, indium, sodium, potassium, silicon, germanium, manganese and all the lanthanoid elements) are present in the form of single-atom molecules, whereas each of the metal elements is liable to aggregate on a substrate of low temperature, forming an independent solid without forming a compound or without diffusing as an impurity element. Further the impurity elements and the other component elements of compound semiconductors, especially nonmetallic elements, are present usually in the form of two-atom molecules (tellurium, nitrogen, iodine, bromine and fluorine), four-atom molecules (arsenic and phosphorus) or multi-atom molecules containing two to eight atoms (sulfur and selenium). Accordingly, during the growth of crystals on the substrate of low temperature, decomposition and incorporation of impurities into the crystals fail to proceed smoothly, permitting occurrence of structural defects (minute twin crystals, crystal grain boundary of small tilt angle, minute island-like growths, etc.). This makes it extremely difficult to grow high-quality single crystals with controlled doping for use in the fabrication of semiconductor devices.
Accordingly, the semiconductor multi-layer epitaxial crystals prepared by the conventional growth method, even when doped with impurities in a controlled fashion so as to exhibit the contemplated conduction type and conductivity, become degraded when heated to a temperature not lower than the growth temperature. For example, it is not exceptional that a low-resistivity semiconductor epitaxial film fully doped with impurities increases in resistivity (ohm-cm) by at least 10 orders of magnitude when treated by heating. Further because the crystals increase in resistivity when heated again close to the growth temperature, formation of the electrode involves the problem that additional ingenuity must be exercised in forming the electrode layer to avoid such a heat treatment. Another problem is encountered in that the electroluminescence device obtained, when driven, thereafter exhibits marked variations in characteristics or deteriorates greatly. These are especially serious problems involved in the use of light-emitting diodes and semiconductor lasers.
Incidentally, although MOCVD as conducted under the irradiation with light is known (Sg. Fujita, A. Tanabe, T. Sakamoto, M. Isemura and Sz. Fujita, Jpn. J. Appl. Phys., 26(1987)L2000; Sz. Fujita, F. Y. Takeuchi and Sg. Fujita, Jpn. J. Appl. Phys. 27(1988) L2019), application of light for MBE is not known.